Signal amplifier of the electron multiplier type



Oct. 12, 1965 c so 3,212,021

SIGNAL AMPLIFIER OF THE ELECTRON MULTIPLIER TYPE Filed Nov. 29. 1960 Z0 F||II||||||I1|||F- I 1' I l L [76.2. 22 R i /5+//Q 204, /.3 1a /4a, L a /5a, /7a. /5w /7a.

United States Patent 3,212,021 SlGNAlL AMPLIFIER 0F THE ELECTRON MULTIPLHER TYPE Melvin L. Erickson, Ivyland, Pa, assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa.,

a corporation of Delaware Filed Nov. 29, 1960, Ser. No. 72,450 2 Claims. (Cl. 33tl42) This invention relates to signal amplifiers and more particularly to signal amplifiers of the electron multiplier type wherein different electron velocities and consequent different transit times between successive electrodes tend to introduce phase variations in the signal.

As is well understood, an electron multipliercomprises a cathode which emits primary electrons, a plurality of spaced secondary electron emissive electrodes known as dynodes, the first of which is impinged by the primary electrons from the cathode, and a collector electrode or anode which receives the secondary electrons from the last secondary electron emissive electrode, and from which the output signal may be derived. Accelerating voltages are applied to the secondary electron emissive electrodes which constitute successive multiplier stages.

It is usually desirable to provide for automatic gain control in the electron multiplier. This can be done by providing at least one resistor in series with one of the secondary electron emissive electrodes, but this gives rise to phase shift in the signal. Where extremely accurate phase of the signal is necessary, the phase shift is highly undesirable.

By way of example, one type of known system in which phase shift of a signal is highly undesirable is a color television receiver which employs a color image-producing cathode ray tube of the index type. In the preferred form of such tube there are color image-producing stripes and index stripes on the screen which are impinged by electrons in the course of scanning of the screen in successive lines transversely of the stripes. The index stripes serve to produce an index signal which is utilized to effect proper time coordination between beam position and modulation, such coordination being essential to proper color rendition in the production of the color image. In such a system the phase of the index signal is representative of beam position, and any appreciable phase shift of the index signal cannot be tolerated as it would cause noticeable error in the color image.

In a preferred form of such system the index stripes emit invisible light in response to electron impingement thereof, and a photomultiplier tube is employed to translate such light into an electrical signal. Such a tube is, of course, an electron multiplier. To provide an index signal of substantially constant amplitude it is desirable to provide for automatic gain control in the photomultiplier as previously mentioned. However, this gives rise to phase variation of the signal which in this instance is highly undesirable.

The principal object of the present invention is to provide a signal amplifier of the electron multiplier type wherein such phase variation is substantially prevented. As hereinbefore mentioned, in a signal amplifier of the electron multiplier type, accelerating voltages are applied to the successive electrodes. This is done conveniently by deriving such voltages from taps on a voltage divider connected to a source of unidirectional voltage. When a resistor is connected in series with one of the secondary electron emissive electrodes, i.e. between that electrode and its divider tap, to provide automatic gain control, the voltage produced across said resistor increases 3,212,021 Patented Oct. 12, 1965 the voltage difference between that electrode and the preceding electrode and decreases the voltage difference between that electrode and the succeeding electrode. By such arrangement, automatic gain control is effeced but transit time variation is introduced which. causes corresponding variation in the phase of the signal.

In accordance with the present invention, the undesirable phase variation is substantially prevented by effecting compensation within the electron multiplier. In one embodiment of the invention this is accomplished by selection of values of the circuit elements associated with the controlled dynode. In another embodiment of the invention the desired cancellation is effected. both by selection of values of said circuit elements and by the provision of an additional circuit element connected to the anode.

The invention may be fully understood from the following detailed description with reference to the accompanying drawing wherein FIG. 1 is a diagrammatic illustration of a signal amplifier according to one embodiment of the present invention;

FIG. 2 is an explanatory diagrammatic illustration; and

FIG. 3 is a diagrammatic illustration of a signal amplifier according to another embodiment of the invention.

Referring first to FIG. 1, there is shown a signal amplifier including a conventional photomultiplier tube 10 such as might be used in a color television system of the type hereinbefore mentioned. As shown, the photomultiplier comprises a photocathode 11, an anode 12, and a series of secondary electron emissive electrodes, i.e. dynodes, there being six such electrodes in the illustrated photomultiplier designated 1 to 6. Accelerating voltages are applied to the dynodes from tap points of a voltage divider comprising series-connected resistors 13 to 19. The voltage divider is connected in series with a source of unidirectional voltage represented as a battery 20.

In operation the photocathode 11 emits primary electrons in response to light impinging thereon. Due to the location of the first dynode 1 and the accelerating voltage applied thereto, the primary electrons impinge said dynode and cause it to emit a multiplied number of secondary electrons. These secondary electrons are drawn successively to the other dynodes under control of their accelerating voltages. In each dynode stage a multiplication takes place so that the operation involves a series of multiplying actions according to the number of dynode stages. The amplified signal is derived from across the inductor 21 connected to the anode 12.

In order to provide for automatic gain control, a resistor 22 is included in the tap connection leading to the dynode 5. As is well understood, this effects automatic gain control by varying the accelerating voltages applied to dynodes 5 and 6. However, it introduces the aforementioned undesirable phase shift in the signal as will now be explained.

Normally the voltage derived from the voltage divider are substantially equal to provide equal voltage differentials between the successive dynodes. However with resistor 22 present, the voltage differentials between dynodes 4 and 5 and between dynodes 5 and 6 are changed by the unidirectional voltage across resistor 22. The latter voltage adds to th voltage across resistor 17 and subtracts from the voltage across resistor 18. It thus increases the voltage differential between dynodes 4 and 5 and decreases the voltage dilferential between dynodes 5 and 6. The effect of this is to decrease the transit time between dynodes 4 and and to increase the transit time between dynodes 5 and 6. The transit time between two successive dynodes is given by the equation K 1 tr w where t is the transit time, K is a constant, d is the distance between the dynodes, and V is the voltage differential between the dynodes. Since d is contant, the transit time varies inversely as the square root of the voltage differential between the dynodes. Thus the transit times between dynodes 4 and 5 and between dynodes 5 and 6 vary in inverse relation to the square root of the voltage differentials. The result of these transit time variations is to introduce phase shift into the signal.

In accordance with this invention, is the embodiment of FIG. 1 the undesired phase variation is substantially prevented by selection of the values of the circuit elements associated with the controlled dynode 5. This will be explained with the aid of FIG. 2 wherein there is shown only the portion of the system which is of concern.

Referring to FIG. 2, the transit times between dynodes 4 and 5 and between dynodes 5 and 6 will be equal if the ratio of the distances al and d is equal to the ratio of the square roots of the voltage differentials between dynodes 4 and 5 and between dynodes 5 and 6. Representing the voltage across resistor 22 as IR, I being the current therethrough and R being the resistance, the voltage differential between dynodes 4 and 5 is E +IR, and the voltage differential between dynodes 5 and 6 is E IR. Thus for equal transit times the following equation must be satisfied If al and d are equal as will normally be the case, solution of the above equation for IR gives To effect equalization of the transit times between dynodes 4 and 5 and between dynodes 5 and 6, resistors 17 and 18 are given values such that E is less than E Then for the nominal value of the current I, resistor 22 is given a value such that the voltage IR thereacross is equal to one-half the difference between the voltages E and E Referring now to FIG. 3, there is shown another embodiment of the invention in which the undesired phase shift is substantially eliminated in a different manner. The voltage across resistor 22a decreases the transit time between dynodes 4 and 5 and causes phase shift. In this instance, however, resistor 18a is made substantially smaller than resistor 17a, e.g. one-half the size of resistor 17a. Therefore, the voltage across resistor 18a is so small that with the subtraction of the voltage across 5&- sistor 22a the voltage on dynode 6 is so small that it is incapable of attracting the electrons which instead are attracted directly to the anode 12, by-passing dynodes 6.

To compensate for or cancel the phase shift produced by resistor 22a, a resistor 23 is included in the connection between anode 12 and ground. This resistor does not affect the automatic gain control action. The voltage across resistor 23 due to current flow therethrough applies a negative bias to the anode and thus increases the transit time between dynode 5 and the anode, compensating for the decreased transit time between dynodes 4 and 5. While thus effecting compensation, resistor 23 does not affect the automatic gain control. By assigning proper values to resistors 22a and 23, the phase shifts caused thereby are made to cancel one another.

By way of example, in a physical embodiment of the arrangement of FIG. 3, the elements have the following values= Kilohms Resistor 13a 30 Resistors 14a to 17a 15 Resistor 18a 7.5 Resistor 19a 10 Resistor 22a 150 Resistor 23 47 It will be observed that in the arrangement of FIG. 3 the by-passing of electrode 6 renders the latter ineffective as a multiplier stage. One might ask therefore why would it not be better to connect the resistor 22a in series with electrode 6, and make resistor 18:: equal to resistor 17a. While this is contemplated by the present invention, it is less desirable than the arrangement of FIG. 3.

While certain embodiments of the invention have been illustrated and described, it will be understood that the invention is not limited thereto but contemplates such modifications and further embodiments as may occur to those skilled in the art.

I claim:

1. In a signal amplifier, an electron multiplier including a series of at least three spaced secondary electron emissive electrodes, means for applying biasing voltages between the first and second of said electrodes and between the second and third of said electrodes, and a resistor connected between said biasing means and said second electrode for providing automatic control of the gain of the amplifier, the parameter being such that where d, is the distance between the first and second electrodes, d is the distance between the second and third electrodes, E is the biasing voltage between the first and second electrodes, E is the biasing voltage between the second and third electrodes, I is the nominal current through said resistor, and R is the resistance of said resistor, whereby the electron transit times between the first and second electrodes and between the second and third electrodes are caused to be substantially equal.

2. In a signal amplifier, an electron multiplier including a series of at least three equally spaced secondary electron emissive electrodes, means for biasing the second of said electrodes positively with respect to the first electrode by a predetermined amount, means for biasing the third electrode positively with respect to the second electrode by a greater amount, and a resistor connector between said biasing means and said second electrode for providing automatic control of the gain of the amplifier, the parameters being such that 2 1 I R- 2 where E is the bias voltage supplied by said first means, E is the bias voltage supplied by said second means, I is the nominal value of current through said resistor, and R is the resistance of said resistor, whereby the electron transit times between the first and second electrodes and between the second and third electrodes are caused to be substantially equal.

References Cited by the Examiner UNITED STATES PATENTS 2,432,654 12/47 Buckbee 313 X 2,553,565 5/51 Ferguson. 2,585,044 2/52 Sanders 330-42 2,798,165 7/57 Neher 250207 2,840,720 6/58 Van Rennes 250207 ROY LAKE, Primary Examiner.

BENNETT G. MILLER, NATHAN KAUFMAN,

Examiners. 

1. IN A SIGNAL AMPLIFIER, AN ELECTRON MULTIPLIER INCLUDING A SERIES OF AT LEAST THREE SPACED SECONDARY ELECTRON EMISSIVE ELECTRODES, MEANS FOR APPLYING BIASING VOLTAGES BETWEEN THE FIRST AND SECOND OF SAID ELECTRODES AND BETWEEN THE SECOND AND THIRD OF SAID ELECTRODES, AND A RESISTOR CONNECTED BETWEEN SAID BIASING MEANS AND SAID SECOND ELECTRODE FOR PROVIDING AUTOMATIC CONTROL OF THE GAIN OF THE AMPLIFIER, THE PARAMETER BEING SUCH THAT 